1. Field of the Invention
The present invention relates to a zoom lens system comprising a flare stop which is displaced in conjunction with zooming.
2. Description of the Prior Art
In designing a zoom lens system, it is a difficult problem to correct coma produced due to excessive intensity of marginal rays. Especially with the recent trend to design wide-angle zoom lens systems, there have been developed so-called retrofocus type zoom lens systems comprising a front negative focusing lens group and a rear imaging lens group. In such a type zoom lens system, pencils having passed through said focusing lens group are diverging and, therefore, paraxial pencil is inevitably incident on high portion of said rear imaging lens group. Further in such a retrofocus type zoom lens system, the pupil is generally located within the rear imaging lens group and, therefore, the offaxial lower pencil to reach the intermediate portion of the image is not limited at the portion where the paraxial ray is high. As a result, optical flare of the offaxial lower pencil is caused to make it difficult to obtain favorable performance of said zoom lens system. That is to say, though it is possible to limit the offaxial pencil having a large angle of field by diameter of the focusing lens group, the pencil having such an angle of field as to reach the intermediate portion of the image cannot be limited by a lens component other than that arranged in the vicinity of the pupil and, therefore, it was impossible to limit said pencil without minimizing the aperture ratio. Furthermore, it is well known that arranging a flare stop at a position far from the pupil is effective to make it possible to effectively control the offaxial pencil. In such a zoom lens system as is provided by the present invention especially of retrofocus type, however, a flare stop which is fixed to the focusing lens group or image-forming variable magnification lens group will be effectively operative at certain magnification levels but excessively limit the pencil to reduce intensity of the marginal pencil or limit the paraxial pencil itself at other magnification level. Such arrangement of a flare stop is therefore undesirable.